A function of a Heating, Ventilation, and Air Conditioning (HVAC) unit is to cool an enclosed space, usually a building. A typical unit can perform two types of cooling, free cooling and mechanical cooling. In free cooling, the unit mixes cooler outdoor air with return air from the building. In mechanical cooling, mechanical components in the unit operate to condition air flowing through the unit. In particular, an evaporator coil absorbs heat from the air flowing past it.
Some units are controlled by an economizer controller, also called an economizer control system. During free cooling, the controller may control the unit based on mixed air, the mixture of outdoor air and return air received by the unit. The controller adjusts the relative amounts of outdoor air and return air, attempting to keep the temperature of the resulting mixed air at a mixed air set-point.
Technically, the term “mixed air” refers to air received by the unit that has not passed the evaporator coil. In contrast, the term “supply air” refers to the air after it passes the evaporator coil. Without mechanical cooling, mixed air temperature and supply air temperature are interchangeable. During mechanical cooling, the evaporator coil reduces the supply air temperature below the mixed air temperature.
In practice, due to space requirements, the temperature of the mixed air is often measured by a sensor located after the evaporator coil. In a typical HVAC unit, the outdoor air and the return air do not mix sufficiently until after the air passes the evaporator coil. If the mixed air temperature sensor were placed before the evaporator coil, either the outdoor air temperature or the return air temperature would dominate the temperature measured by the sensor. Therefore, to accurately measure the mixed air temperature, the mixed air temperature sensor is located after the evaporator coil.
Despite its name then, the “mixed air” temperature sensor really measures the temperature of supply air. The position of the mixed air temperature sensor creates an issue when free cooling and mechanical cooling are performed together. The evaporator coil tends to cool the air below the mixed air set-point. In response to the low mixed air temperature, the controller attempts to warm the mixed air. As a result, the controller signals the unit to stop using outdoor air to cool the building.
It would be desirable if a solution existed that would allow the unit to continue to receive outdoor air when free cooling is available and the unit is performing mechanical cooling. It would further be desirable if the solution could be implemented with only minimal modifications to an existing HVAC unit and HVAC controller.